The maintenance crew is still needed to keep the engines running. It's just that it can be piloted remotely. This, however, is nothing new technologically speaking. We could very well take away pilots from commercial airliners but it will never happen, or not soon enough. Because a human in control gives a nice illusion of safety.

When it comes to freighters, the cost of a captain or even a crew is negligible. It pales in comparison to the cost of the ship and the goods it carries. Also, insurance companies would not go down well with this.